It is common knowledge that propeller systems, for example, aircraft propeller systems, pushers, tractors, and tail rotors, generate a noise signature. It is generally desirable to reduce the noise signature of propellers. As noted in U.S. Pat. No. 7,992,674, the disclosure of which is incorporated herein, noise generated by propeller systems has both broadband and tonal components. Tonal noise results from propeller blade interactions with time-invariant flow distortions. When spectrally dominant, blade tones are of primary concern in noise control applications due to their particular annoyance.
The noise problem is particularly acute for surveillance aircraft and drones that must avoid detection or creating a nuisance near populated areas. Several conventional techniques have been used to reduce propeller noise, but they all tend to reduce the efficiency of the propeller. Some conventional noise control techniques include, increasing the number of blades, slowing the rotating speed of the propeller, and shaping the blades for minimum noise creation.
Prior approaches used to reduce blade tone sound pressure levels (SPLs) have utilized both active and passive noise control methods. Passive blade alterations, such as rotor/stator spacing in axial fans, leaning, sweeping or contouring, numbering, and irregular circumferential blade spacing, have been demonstrated effective for fan noise reduction. Few passive approaches have demonstrated the ability to reduce blade tone noise locally in the blade region with minimal impact on fan efficiency.
The concept of noise cancellation by introducing secondary sources or resonant systems is a well understood and implemented concept. Obstructions, such as cylindrical rods, can be placed in the near field of a rotor to generate an anti-phase secondary sound field that can then be tuned to reduce blade tone noise. However, difficulty in tuning the response of these interactions often limits their usefulness. Active noise control approaches have been used for blade tone noise reduction, e.g., introducing active secondary sources into the existing sound field of an axial fan. Conventional active approaches have used loudspeaker arrays to reduce levels of fan noise propagating down a duct. Due to the associated weight and non-compactness of loudspeakers, piezoelectric actuators have been used more recently as acoustic transducers imbedded into the stator vanes of axial fans to reduce tonal noise propagations. Air injections, either positioned to generate secondary sources through interaction with the rotor blades or used to improve flow non-uniformities generated by a body in a flow field, have been shown to reduce tonal noise. These approaches have proven effective in a laboratory setting, but are generally prohibitively expensive and potentially unreliable in most actual axial fan applications.
The first known implementation of flow-driven resonator source was to generate a canceling sound field that reduced fan noise generated by a centrifugal blower. More recently, as disclosed in U.S. Pat. No. 7,992,674, a method of using resonators as flow driven secondary sources has been developed for axial fans. This method behaves as a quasi-active source cancellation wherein fluid flow interacts with a resonator as a means of generating an acoustic source.
The application to propellers, especially open (non-shrouded) propellers, and specifically aircraft propellers has not been addressed for various reasons. As will be discussed in more detail below, when cancelling propeller tones with an acoustic resonator, the combination of primary and secondary sound sources tends to create spatial patterns of quiet zones and loud zones. Aircraft applications are somewhat unique in that the aircraft, in flight, is generally in an acoustic free-field (i.e., no reflective bodies nearby). In fact, the sound that is projected upward, away from the ground, is usually of no concern. Propellers also have a very directional sound field. As a result, a noise “reduction” solution can be applied that targets the directional sound field of the propeller but may increase the sound in some directions.
A key barrier to implementing sound cancellation methods in aircraft is the added weight. For previous systems, the weight penalty has generally been too high for the acoustic performance gains.
It may be desirable to provide a propeller system with a flow-driven acoustic resonator for modifying or shaping the sound field of a propeller system of an aircraft. It may be desirable to provide a system where the propeller is unaltered and the potential noise reduction in a desired direction is significant. It may also be desirable to provide a propeller sound field modification system that can be integrated into existing aircraft structures such that any additional weight or drag due to the implementation will be relatively nominal. It may be desirable to provide a secondary sound source that accommodates relative motion of the propeller to the aircraft structure with flexible components.